1. Field of the Invention
This invention relates to fabrication of integrated circuits, and more particularly to a method of forming a mark in an IC-fabricating process, wherein the mark is defined by two exposure steps and can serve as a basis of an overlay mark.
2. Description of the Related Art
Through an IC-fabricating process, various marks are formed on the wafer for different purposes. For example, to check the alignment accuracy between the patterns of lower and upper wafer layers that is more and more important as the linewidth gets smaller increasingly, a wafer is formed with many overlay marks in some non-die areas. A conventional overlay mark is the so-called box-in-box (BIB) overlay mark, but such overlay marks are too sensitive to certain factors other than the alignment so that the alignment accuracy cannot be checked accurately. Hence, the overlay mark of AIM (advanced imaging mark) type is provided in the prior art.
Referring to FIG. 1, a conventional AIM-type overlay mark 100 is disposed in a non-die area of the wafer (not shown) including four adjacent regions 102-108, which are arranged in a 2×2 array and include a first region 102 and a second region 104 arranged diagonally and a third region 106 and a fourth region 108 arranged diagonally, and includes a portion of the lower layer defined by a lithography process for defining the die patterns of the lower layer and a patterned photoresist layer formed in a lithography process for defining the die patterns of the upper layer. The portion of the lower layer includes a first set of x-directional linear patterns 110 arranged in the y-direction in one half of the first region 102, a second set of x-directional linear patterns 112 arranged in the y-direction in one half of the second region 104, a first set of y-directional linear patterns 114 arranged in a x-direction in one half of the third region 106, and a second set of y-directional linear patterns 116 arranged in the x-direction in one half of the fourth region 108. The patterned photoresist layer includes a first set of x-directional photoresist bars 118 arranged in the y-direction in the other half of the first region 102, a second set of x-directional photoresist bars 120 arranged in the y-direction in the other half of the second region 104, a first set of y-directional photoresist bars 122 arranged in the x-direction in the other half of the third region 106, and a second set of y-directional photoresist bars 124 arranged in the x-direction in the other half of the fourth region 108.
The linear patterns 110-116 and photoresist bars 118-124 are designed such that when the lower layer is fully aligned with the upper layer, the central line of respective central lines of the first set of x-directional linear patterns 110 and the second set of x-directional linear patterns 112 coincides with that of respective central lines of the first set of x-directional photoresist bars 118 and the second set of x-directional photoresist bars 120, and the central line of respective central lines of the first set of y-directional linear patterns 114 and the second set of y-directional linear patterns 116 coincides with that of respective central lines of the first set of y-directional photoresist bars 122 and the second set of y-directional photoresist bars 124.
To check the alignment, the y-coordinate “y1a” of the central line of the first set of x-directional linear patterns 110, the y-coordinate “y1b” of the central line of the second set of x-directional linear patterns 112, the x-coordinate “x1a” of the central line of the first set of y-directional linear patterns 114, the x-coordinate “x1b” of the central line of the second set of y-directional linear patterns 116, the y-coordinate “y2a” of the central line of the first set of x-directional photoresist bars 118, the y-coordinate “y2b” of the central line of the second set of x-directional photoresist bars 120, the x-coordinate “x2a” of the central line of the first set of y-directional photoresist bars 122 and the x-coordinate “x2b” of the central line of the second set of y-directional photoresist bars 124 are derived at first.
The method of deriving the x- and y-coordinates is exemplified by the following process of deriving x1a that is shown in FIG. 2. The first y-directional linear patterns 114 are scanned by a light beam (not shown) in the direction 200 to obtain a reflectivity curve 202, and respective x-coordinates of the six first y-directional linear patterns 114 are determined based on the reflectivity curve 202. When the linear patterns 110-116 are, for example, trenches in the portion of the lower layer, x1a is calculated as the average of the x-coordinates x1a1, x1a2, x1a3, x1a4, x1a5 and x1a6 of the six locally minimal points of the reflectivity curve 202.
Then, the x-directional alignment error of the die patterns of the upper layer with those of the lower layer near the overlay mark is calculated as “(x2a+x2b)/2−(x1a+x1b)/2”, and the y-directional alignment error of the die patterns of the upper layer with those of the lower layer near the overlay mark is calculated as “(y2a+y2b)/2−(y1a+y1b)/2”. After the x-directional alignment errors and y-directional alignment errors at different areas of the wafer are determined using the overlay marks thereat, overlay analyses can be done for better control of the exposure system.
Moreover, when the lower layer is defined by two exposure steps, in the prior art, two above overlay marks have to be formed for the two exposure steps respectively, so that the x-directional and y-directional alignments of the die patterns defined by the first exposure step as well as those of the die patterns defined by the second exposure step with the die patterns of the upper layer can be checked.
However, when one exposure step uses X-dipole off-axis light to define patterns requiring higher resolution in the x-direction and the other uses Y-dipole off-axis light to define patterns requiring higher resolution in the y-direction, each set of x-directional linear patterns arranged in the y-direction in the overlay mark 100′ for the X-dipole exposure merge together, as shown in FIG. 3(a), and each set of y-directional linear patterns arranged in the x-direction in the overlay mark 100″ for the Y-dipole exposure step merge together, as shown in FIG. 3(b). Therefore, a half area of each overlay mark cannot be utilized in the alignment check and is wasted.